Measuring surface longshore currents with an optical technique Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/hh63sz965

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  • Longshore currents are frequently occurring phenomena capable of transporting beach sediments, causing accretion and erosion of the shoreline. Forcing mechanisms are understood and well modeled for cases of alongshore homogeneous and monotonic bathymetry, yet the extension of these models to complex and irregular bathymetry sometimes fails. To test and improve these models, observational data over large spatial and temporal scales and from a variety of beaches and forcing conditions is necessary. In situ instruments have the drawback of being costly and difficult to operate for very long periods (months to years) over wide areas (hundreds of meters) on a routine basis, due to the frequent care and attention they demand while in the high energy environment of the surf zone. We present an optical method (Optical Current Meter or OCM) to measure the longshore component of surface currents in the nearshore by measuring the alongshore drift of persistent sea foam in the surf zone. The method is based on short time series of video data collected from an alongshore array of pixels. These space-time data are first Fourier transformed to a frequency-wavenumber spectrum, and finally to a velocity spectrum. A model of the velocity spectrum, including energy from noise and the expected energy from drifting foam traces, is fitted to the observed spectrum to estimate the foam drift velocity. The 95% confidence interval around the velocity estimate and other measures of the input and output data quality are calculated. Tests with synthetic data were performed to evaluate the ability of the OCM to measure the mean current in the presence of waves and wave orbital velocity contamination. Synthetic tests showed that, over a range of mean velocities, orbital velocities, and varying time series lengths used in the analysis, the OCM accurately estimated the mean current (with maximum RMS error of 0.033 m/s for an 8 s window of analysis). Approximately one month of video data from the 1997 SandyDuck field experiment was collected from an alongshore array of pixels collocated in the video image with an in situ bidirectional electromagnetic current meter, and was used to ground truth test the OCM. Mean longshore currents were estimated from each instrument for overlapping data records. The root-mean-square error between the two records is 0.11 m/s. Additionally, a linear regression showed the gain between the two instruments to not be statistically different from one. The differences between the surface and interior measurements were compared to forcing mechanisms that may cause surface velocity shear. A linear regression of aurface velocity shear to alongshore wind stress was well correlated for cases when waves were arriving from the south with no significant correlation for waves arriving from the north. We attribute this to an increased sensitivity to misregistration errors when viewing waves from the north. This technique can be applied to study large-scale coastal behavior and to measure shear waves of longshore currents.
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